How Deadly Assassin Bugs Sneak Up on Spiders

Scientists discover how hungry giraffe assassin bugs muffle their rampage through a web to grab the unwitting occupant.

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The poorly known giraffe assassin bug turns the tables on spiders: by eating them.

Photograph courtesy Fernando Soley

PUBLISHED October 25, 2016

Armed with fangs, venom, and exquisitely acute senses, spiders are the supreme predators of the creepy-crawly world. But they’re no match for a killer bug that can outwit a spider’s powers of vigilance, new experiments show.

Giraffe assassin bugs have learned to squelch the vibrations they make as they tear through webs to reach their eight-legged prey, according to a study published in this week’s Royal Society Open Science. The bug even turns the wind to its advantage, deliberately targeting spiders when a blowing breeze will mask its approach.

“For the spider, the bug must be the bogeyman,” says study author Fernando Soley, who did the research at Australia’s Macquarie University and is now at the University of Costa Rica. “Because it hits you out of nowhere, and it’s scary.”

Assassin Bug vs. Bats

Watch: Another species of assassin bug preys on vampire bats.

Tough Prey

Anything that wants to eat a spider is aiming high. A spider web is a sprawling, ultrasensitive intelligence-gathering system, transmitting signals about where to find that ensnared gnat or whether sector 6A needs patching. A spider can detect thread movements roughly a millionth of an inch in its web, says biologist Beth Mortimer of the University of Oxford and the University of Bristol, who was not involved with the study.

Even so, giraffe assassin bugs (Stenolemus giraffa)—named for their long “necks” that make up one-third to one-half their body length—manage to dine on spiders. Soley, who has studied the insects for years, noticed that spiders were usually oblivious to assassin bugs, even as they literally ripped apart webs to reach their victims.

To understand the bugs’ M.O., Soley wiggled into rock crevices in Australia to scoop up assassin bugs, arduous work that netted him 10 bugs on a good day. In the lab, he coaxed spiders into spinning draglines and attached the threads to a wooden frame to make an artificial web. He lured assassin bugs into the simulated webs using daddy longlegs as decoys.

By measuring web vibrations with a laser-based tool, Soley learned that the bugs almost always used what he dubbed the “cautious” method for reaching a spider. The insect would grab a strand of webbing and stretch the silk in opposite directions, as if pulling taffy. After the thread broke, the bug held onto the loose ends. The result was avoiding rapid recoils and reducing vibrations in the web.

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A giraffe assassin bug (top) stalks its prey: a spider in its web.

Photograph courtesy Fernando Soley

“It’s a very clever, subtle way to get at a spider,” says spider expert William Eberhard of Louisiana State University and the Smithsonian Tropical Research Institute, who was not involved with the study. “I personally would’ve thought it wouldn’t work, but obviously it does.”

Blowing in the Wind

Soley also aimed a fan at the webs some of the time and found that the bugs preferred to break threads when the air was moving. Wind seems to ruffle webbing and spider vibratory sensors alike, making it harder for spiders to detect intruders. Soley has also found that the bugs space out their thread-breaking over time and will back off if a spider starts to investigate.

Both Eberhard and Mortimer are convinced by the new results, which put hard numbers on observations Soley has made in the lab and in the wild. Even though spiders are highly attuned to motion in their webs, the bug manages to alter its vibratory “footstep” in the webbing “so it is not as easily classified as ‘silk thread breaking,’” Mortimer says via email.

Once the bug succeeds in getting close to a spider, it stabs the arachnid with its needlelike mouthpiece. The spider starts to shake—perhaps the bug injects a toxin, Soley says—and then dies an ignominious death in what amounts to very bad spider P.R.